arXiv subjects

Baltej Singh

Publications and source records attributed to Baltej Singh.

23 records · Page 2Linked to original sources

arXiv2018

Ab-initio Molecular Dynamics study of 1-D Superionic Conduction and Phase Transition in \b{beta}- Eucryptite

$β$- Eucryptite (LiAlSiO4) is a potential electrolyte for Li- ion battery due to its high Li- ion conductivity and very small volume thermal expansion coefficient. We have performed ab-initio molecular dynamics simulations of $β$- Eucryptite to study the origin of high temperature superionic phase transition in this material. The simulations are able to provide the microscopic understanding of the one -dimensional superionicity that occurs along the hexagonal c-axis and is associated with the order-disorder nature of the phase transition. The Li ionic conductivity is found to enhance due to the anisotropic negative thermal expansion along hexagonal c-axis. The introduction of defects in the crystal like, excess Li in interstitial sites, Li vacancy and O vacancy are found to significantly increase the ionic conductivity and hence might reduce the temperature of the superionic phase transition in this material.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous Thermal Expansion, Negative Linear Compressibility and High-Pressure Phase Transition in ZnAu2(CN)4: Neutron Inelastic Scattering and Lattice Dynamics Studies

We present temperature dependent inelastic neutron scattering measurments, accompanied byab-initio calculations of phonon spectra and elastic properties as a function of pressure to understand anharmonicity of phonons and to study the mechanism of negative thermal expansion and negative linear compressibility behaviour of ZnAu2(CN)4. The mechanism is identified in terms of specific anharmonic modes that involve bending of the Zn(CN)4-Au- Zn(CN)4 linkage. The high-pressure phase transition at about 2 GPa is also investigated and found to be related to softening of a phonon mode at the L-point at the Brillouin zone boundary and its coupling with a zone-centre phonon and an M-point phonon in the ambient pressure phase. Although the phase transition is primarily driven by a L-point soft phonon mode, which usually leads to a second order transition with a 2 x 2 x 2 supercell, in the present case the structure is close to an elastic instability that leads to a weakly first order transition.

cond-mat.mtrl-sci↗

arXiv2017

Anomalous Lattice Behavior of Vanadium Pentaoxide (V2O5): X- Ray Diffraction, Inelastic Neutron Scattering and ab-initio Lattice Dynamics

We present the structural and dynamical studies of layered vanadium pentaoxide (V2O5). The temperature dependent X-ray diffraction measurements reveal highly anisotropic and anomalous thermal expansion from 12 K to 853 K. The results do not show any evidence of structural phase transition or decomposition of α-V2O5, contrary to the previous transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) experiments. The inelastic neutron scattering measurements performed up to 673 K corroborate the result of our X-ray diffraction measurements. The analysis of the experimental data is carried out using ab-initio lattice dynamics calculation. The important role of van der-Waals dispersion and Hubbard interactions on the structure and dynamics is revealed through the ab-initio calculations. The calculated anisotropic thermal expansion behavior agrees well with temperature dependent X- ray diffraction. The mechanism of anisotropic thermal expansion and anisotropic linear compressibility is discussed in terms of calculated anisotropy in Grüneisen parameters and elastic coefficients. The calculated Gibbs free energy in various phases of V2O5 is used to understand the high pressure and temperature phase diagram of the compound. Softening of elastic constant (C66) with pressure suggests a possibility of shear mechanism for α to \b{eta} phase transformation under pressure.

cond-mat.mtrl-sci↗

arXiv2016

Role of Phonons in Li-Diffusion, Thermal Expansion and Phase Transitions in beta-eucryptite: Inelastic Neutron Scattering and Lattice Dynamics Studies

Beta eucryptite (LiAlSiO4) shows one-dimensional super-ionic conductivity as well as anisotropic thermal expansion behavior. We have performed inelastic neutron scattering measurements in beta eucryptite over 300 to 900 K and calculated the phonon spectrum using ab initio density functional theory method. The calculated energy profile for cooperative lithium ion displacements indicates preferential movement of Li ion along the hexagonal c-axis in the high temperature phase. However, the energy barrier for Li ion diffusion is significantly reduced when a Schottky defect is introduced in the crystal. Further, the anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10 meV have large negative values and govern the negative thermal expansion behavior both along the a and c axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30 to 70 meV are responsible for positive thermal expansion in the a-b plane and negative expansion along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous thermal expansion behavior. We extend the study to discuss the relationship of the soft phonons in the Brillouin zone with the observed high-pressure and high-temperature phase transitions as reported in the literature.

cond-mat.mtrl-sci↗

arXiv2015

Lattice Dynamics and Thermal Expansion Behavior in Metal Cyanides, MCN (M=Cu, Ag, Au): Neutron Inelastic Scattering and First Principles Calculations

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along c axis in the Cu and Ag compounds, while the same modes contribute to positive thermal expansion in the Au compound. Several low energy transverse modes lead to positive thermal expansion along a and b axis in all the compounds. The calculated elastic constants and Born effective charges are correlated with the difference in nature of bonding among these metal cyanides.

cond-mat.mtrl-sci↗